Energy Release from Fats

Overview of Fat Metabolism

Fats are a major source of energy in the human body, especially during periods of fasting or prolonged exercise. The breakdown of triglycerides releases large amounts of ATP, making fats an efficient energy store.

• Triglycerides are broken down by lipases into glycerol and three fatty acids.

• Glycerol (a 3-carbon molecule) can be converted into dihydroxyacetone phosphate, which enters glycolysis and oxidative phosphorylation.

• Fatty acids undergo beta-oxidation in the mitochondria, where two-carbon units are removed sequentially to form acetyl CoA.

• Each acetyl CoA enters the Krebs cycle (citric acid cycle), producing NADH, FADH2, and ATP.

ATP Yield from Palmitic Acid (16-carbon fatty acid):

• Beta-oxidation produces 8 acetyl CoA molecules.

• Each acetyl CoA yields 10 ATP via the Krebs cycle and electron transport chain.

• Additional ATP is produced from NADH and FADH2 generated during beta-oxidation.

Net ATP Yield Calculation:

• 10 ATP from each 2C unit × 8 = 80 ATP

• 4 ATP from (NADH + FADH2) × 7 = 28 ATP

• Activation cost subtracted; net yield = 106 ATP per palmitic acid

• Complete oxidation of a molecule of palmitic acid yields 334 ATP (including all steps)

Equation:

Energy Storage: Function of Adipocytes

Role of Adipocytes in Energy Homeostasis

Adipocytes, or fat cells, are specialized for the storage and release of energy in the form of triglycerides. They play a crucial role in maintaining energy balance during periods of food intake and fasting.

• Adipocytes store fat during periods of food abundance and release it during fasting or exercise.

• Fat is stored in the form of triglycerides within lipid droplets in adipocytes.

• During exercise, stored fat is mobilized to provide energy for muscle activity.

Body Energy Stores:

• Average adult body contains about 15 kg of fat (~140,000 kcal).

• Protein stores: 9 kg = 38,000 kcal

• Carbohydrate stores: 0.5 kg = 2,000 kcal

Example: During prolonged fasting, the body relies primarily on fat stores for energy, with protein and carbohydrate reserves being much smaller.

Metabolic Pathways: Carbohydrates vs. Fats

Comparison of Energy Metabolism

Carbohydrates and fats follow distinct metabolic pathways for energy production. The body’s choice of substrate depends on nutritional state, activity level, and tissue type.

• Carbohydrates are metabolized via glycolysis to produce ATP rapidly, especially important for the brain and nervous tissue.

• Fats are metabolized via beta-oxidation, which is slower but yields more ATP per molecule.

• During the fed state, nutrients are stored; during fasting or exercise, they are oxidized for energy.

Key Points:

• Glucose is stored as glycogen or converted to fat.

• Fatty acids are stored as triglycerides or oxidized to acetyl CoA.

• Acetyl CoA enters the Krebs cycle, producing carbon dioxide and water as waste products.

Energy Content of Nutrients

Calorimetry and Metabolizable Energy

The energy content of nutrients is measured in kilocalories (kcal) per gram. Different macronutrients provide varying amounts of energy when metabolized.

*The difference in protein values is due to the energy content of urea, which cannot be further metabolized in the body.

Key Terminology in Energy Metabolism

Important Terms and Definitions

Understanding the following terms is essential for studying energy metabolism:

• Formation/creation: The process of synthesizing new molecules.

• Breakdown/destruction: The process of degrading molecules to release energy.

• New: Refers to newly synthesized compounds.

• Phosphatase: An enzyme that removes a phosphate group from a molecule.

• Glycogenesis: Formation of glycogen from glucose.

• Glycogenolysis: Breakdown of glycogen to release glucose.

Energy Supply and Exercise

Rate of Energy Supply During Physical Activity

The rate at which energy is supplied to tissues depends on the intensity and duration of exercise, as well as the substrate being used.

• Carbohydrates provide rapid energy for high-intensity exercise.

• Fats are used for sustained, lower-intensity activity.

• During increasing workloads, the body transitions from carbohydrate to fat metabolism, reaching a steady state until the workload changes.

Equation:

RER values indicate the predominant fuel source: 1.0 for carbohydrates, ~0.7 for fats.

Summary Table: Metabolic Pathways of Carbohydrates and Fats

Comparison of Pathways and End Products

Additional info: Some context and terminology were inferred from standard physiology textbooks to clarify fragmented notes and images.